Shocking evidence of conditions akin to slavery on trawlers that provide fish for European dinner tables has been found in an investigation off the coast of west Africa.

Forced labour and human rights abuses involving African crews have been uncovered on trawlers fishing illegally for the European market by investigators for an environmental campaign group.

The Environmental Justice Foundation found conditions on board including incarceration, violence, withholding of pay, confiscation of documents, confinement on board for months or even years, and lack of clean water.

The EJF found hi-tech vessels operating without appropriate licences in fishing exclusion zones off the coast of Sierra Leone and Guinea over the last four years. The ships involved all carried EU numbers, indicating that they were licensed to import to Europe having theoretically passed strict hygiene standards.

"We didn't set out to look at human rights but rather to tackle the illegal fishing that's decimating fish stocks, but having been on board we have seen conditions that unquestionably meet the UN official definition of forced labour or modern-day slavery," EJF investigator Duncan Copeland said. A report on the abuses is published by the foundation today.

Its photographs and film of the areas in which the crews were working and sleeping show quarters with ceilings less than a metre high where the men cannot stand up. Temperatures in the fish holds on some vessels where men were being required to sort, process and pack fish for lucrative European and Asian markets were 40 to 45 degrees, with no ventilation, On some vessels the crews of up to 200 had little access to clean drinking water.

The trawlers have mostly been identified engaging in pirate fishing off west Africa. Many of the men on board have been recruited from the area around the Senegalese capital, Dakar. Others have been recruited from rural areas of Asia, including China and Vietnam, by agents.

According to a recent estimate illegal fishing accounts for between 13% and 31% of total catches worldwide each year, but accurate figures are hard to come by.